The present invention relates to a process for the deposition of protective coatings on complex shaped Si-based substrates which are used in articles subjected to high temperature, aqueous environments.
Ceramic materials containing silicon have been proposed for structures used in high temperature applications, for example, gas turbine engines, heat exchangers, internal combustion engines, and the like. A particularly useful application for these materials is in gas turbine engines, which operate at high temperatures in aqueous environments. It has been found that such silicon containing substrates can recede and lose mass as a result of a formation volatile Si species, particularly Si(OH)x and SiO when exposed to high temperature, aqueous environments. For example, silicon carbide, when exposed to a lean fuel environment of approximately 1 ATM pressure of water vapor at 1200° C., will exhibit weight loss and recession at a rate of approximately 6 mils per 1000 hrs. It is believed that the process involves oxidation of the silicon carbide to form silica on the surface of the silicon carbide, followed by reaction of the silica with steam to form volatile species of silicon such as Si(OH)x.
Suitable coatings for articles containing silicon based substrates which are employed in the environments claimed above are well known in the art. See for example U.S. Pat. Nos. 5,305,726; 5,869,146; 6,284,325; 6,296,941; 6,352,790; and 6,387,456. The prior art described in the foregoing patent documents fail to teach processes for applying protective coatings to complex shaped parts such as, for example, integral vane rings and integrally bladed rotors. Typically, such complex shaped parts have been coated by a chemical vapor deposition (CVD) process; however, such CVD processes are limited in that they are only applicable to simple oxide coatings, are limited in terms of thickness and uniformity of the coating layer, and are expensive.
Naturally, it would be highly desirable to provide improved processes for the deposition of protective coatings on complex shaped Si-based structures.
Accordingly, it is a principle object of the present invention to provide a non-line-of-sight process as aforesaid which is efficient and inexpensive when compared to prior art processes.
It is a further object of the present invention to provide processes as aforesaid which include electrophoretic deposition (EPD) and electrostatic deposition (ESD) of protective coatings.
It is a still further object of the present invention to provide an EPD process which is efficient for the deposition of protective coatings on complex shaped Si-based structures.